Electro-thermal relay actuator

ABSTRACT

A relay in which the actuator is a bimetallic strip to which is applied a thin coating of electrical insulation material. Over the layer of insulation material is applied a layer of electrically resistive material which heats up in response to an electric current. A movable contact in the form of a flat spring is supported between two spaced pivot points that are more closely spaced than the length of the spring. The bimetal is linked to the spring such that bending of the bimetal with heating causes the movable contact spring to snap over center between the pivots.

United States Patent 1191 Bell 1451 Oct. 15,1974

1 1 ELECTRO-THERMAL RELAY ACTUATOR 337/85, 94, 1 02, 103, 107, 377, 365;200/DIG. 28; 338/307, 308, 314; 219/511 [56] References Cited UNITEDSTATES PATENTS 2,518,941 8/1950 Satchwell et al 337/107 2,793,268 5/1957Franklin 337/102 X 3,019,319 1/1962 Anderson... 3,033,959 5/1962Flanagan... 3,274,359 9/1966 Riebs 337/107 3,316,374 4/1967 NClSOn337/107 3,579,167 5/1971 0111616 337/107 x FORElGN PATENTS ORAPPLlCATlONS 1,151,859 5/1969 Great 131116111 337 107 398,476 7 1924Germany 200/010. 28

Primary ExaminerA. J. Grimley Attorney, Agent, or Firm-Christie, Parker& Hale [57] ABSTRACT A relay in which the actuator is a bimetallic stripto which is applied a thin coating of electrical insulation material.Over the layer of insulation material is applied a layer of electricallyresistive material which heats up in response to an electric current. Amovable contact in the form of a flat spring is supported between twospaced pivot points that are more closely spaced than the length of thespring. The bimetal is linked to the spring such that bending of thebimetal with heating causes the movable contact spring to snap overcenter between the pivots.

18 Claims, 7 Drawing Figures ELECTRO-THERMAL RELAY ACTUATOR FIELD OF THEINVENTION This invention relates to electrical relays, and moreparticularly to a thermo-electric actuator for operating a relay.

BACKGROUND OF THE INVENTION Conventional relays are magneticallyactuated, that is, an electrical current applied to a relay coilmagnetically attracts an armature to which are linked the movingcontacts of the relay switch Relays are used in increasing numbers andmany diverse applications, such as in automobiles and home applianceswhere cost is a major factor. Because they require a coil winding offine copper wire, the cost of relays is relatively expensive. Moreoverthe high inductance of the coil presents electrical problems, such asinduced high-voltage spikes and stray magnetic fields which must beshielded.

The present invention provides an improved relay in which the actuatoris thermo-electrically controlled rather than electromagneticallycontrolled. The relay is substantially less expensive to manufacture andavoids the electrical problems inherent in the conventionalelectromagnetically actuated relay.

Thermo-electric actuated switch devices are well known in which theheating and cooling of a bimetallic strip operates a switch, forexample. The use of an electrical heating element to control thetemperature of the bimetal strip has also heretofore been proposed.Generally the heater has been in the form of a heating unit which isseparate from the bimetal strip. The thermal lag is rather substantial,providing a relatively slow response time to the control of currentthrough the heater. While it has been proposed to mount a heaterdirectly on the bimetal element, such known arrangements have requiredspecial forming of the bimetal strip to mount the heating element andhave not provided uniform heat distribution over the entire surface ofthe bimetallic strip.

SUMMARY OF THE PRESENT INVENTION The present invention provides animproved thermoelectrical actuator for operating a switch to provide arelay. In brief, the invention provides an actuator mechanism whichcomprises a bimetallic strip having a thin coating of electricalinsulation on one surface of the strip, the insulation being overlayedwith a thin coating of electrically resistive material. The bimetalstrip engages a flat spring anchored in bowed condition between twospaced pivots. Movement of the bimetal strip causes the flat spring tosnap over center to open and close a pair of switching contacts.

BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding ofthe invention reference should be made to the accompanying drawings,wherein:

FIG. 1 is a cross-sectional view of a relay utilizing a thermal actuatoraccording to the present invention;

FIG. 2 is a sectional view taken substantially along the line 22 of FIG.1;

FIG. 3 is a top view of the relay of FIG. 1;

FIG. 4 is an enlarged perspective view of a modified thermal actuatordesigned for the relay of FIG. I;

FIG. 5 is a further modification to the relay of FIG. 1 having acontrolled response time; and

FIG. 6 is an alternative embodiment of a relay using a thermal actuator.

DETAILED DESCRIPTION Referring to FIGS. 1-3 in detail, the numeral 10indicates generally the housing or case for the relay, which ispreferably molded of a plastic material that provides both a goodthermal and electrical insulation. The case is molded with a shelf 12 atone end to which is secured a flat spring 14. The spring 14 ispreferably secured in position on the shelf 12 by a pair of screws 16.The flat spring 14 extends in cantilever fashion from the shelf 12longitudinally of the housing 10. As best seen in FIG. 2, the flatspring 14 is provided with a step reduction in width adjacent one end,as indicated at 18. The reduced width portion terminates in anelectrical contact 22. The step 18 engages a pivot member 24 which issecured to and projects up from the bottom of the housing 10. The pivothas an opening 26 through which the reduced width portion 20 of the flatspring 14 projects. The flat spring is bent upwardly slightly at theedge of the shelf 12, as indicated at 28, so that the flat spring bowsupwardly between the shelf 12 and the pivot 24. In this condition, themoving contact 22 on the end of the flat spring 14 engages a fixedcontact 30 mounted on the bottom of the case 10 by an electricallyconductive contact support member 32 which is electrically connected tothe outside of the case by a terminal 34. The flat spring 14 is alsoformed with a terminal portion 36 which extends outside the housing orcase 10 at the shelf 12. Thus the contacts 22 and 30 provide a normallyclosed current path between the terminals 34 and 36.

An actuator is provided for operating the switch and to open thenormally closed contacts. The actuator comprises a bimetallic strip 40which is supported on a shelf 42 at the opposite end of the housing fromthe shelf 12. The bimetallic strip is secured to the shelf 42 by meansof a pair of screws 44. The bimetallic strip 40 is substantially flatand extends in cantilever fashion above the flat spring 14. The free endof the bimetallic strip 40 is bent downwardly, as indicated at 46, inposition to press against the top of the flat spring 14 at a point whichis nearer to the shelf 12 than to the pivot member 24.

Electrical heating of the bimetal element 40 is provided by coating thetop surface of the bimetallic element with a very thin layer of ahigh-temperature plastic material, such as a polymide insulation capableof withstanding temperatures up to 600F. A thin sheet or foil ofnichrome material is then bonded or cemented to the insulating coating.The nichrome layer is then formed into a serpentine-shaped currentconductive path by an etching process, such as is well known in theprinted circuit art. The current path provided by the nichrome materialterminates in a pair of terminals 48 which can be connected across asuitable power source such as a battery (not shown). By passing acurrent through the nichrome material, heat is applied uni formly overthe entire surface of the bimetallic element causing it to deflect so asto move the end 46 downwardly against the flat spring 14.

By making the spring constant of the flat spring 14 higher adjacent theshelf 12, as by increasing the width of the spring 14 over a portion ofits length, indicated at 50, the downward movement of the flat springunder the urging of the bimetallic element when heated, causes the flatspring to snap into an S-shaped configuration, as shown by the dottedline in FIG. 1. As a result, the moving contact 22 snaps upwardly fromthe position shown in FIG. 1 against a normally open electrical contact52. The contact 52 is securely mounted within the case by a conductivesupport member 54 having a terminal 56 extending outside of the case.

On disconnecting the electrical heater from the power source, thebimetallic strip, as it cools, returns to its original position, asshown in FIG. 1. The cantilever mounting of the flat spring 14 causesthe spring to return to its initial shape in which it snaps from the S-shape shown by the dotted line back to its initial bowed shape, therebyreturning the moving contact 22 from the normally open contact 52 to thenormally closed contact 30.

The arrangement as thus far described is relatively fast acting becausethere is almost instantaneous thermal transfer from the heater to thebimetallic element over substantially the full area of the bimatallicelement. The relay switching mechanism provides a snap action, givinglonger contact life. The particular design insures that the contactforce remains high until the moment that switching occurs, therebyfurther reducing contact wear and burning due to arcing.

Referring to the embodiment of FIG. 4, the bimetallic element may beconstructed in two sections, 60 and 62. In section 62, the metallicmaterials forming the two layers of the bimetallic strip are reversed soas to provide deflection in the opposite direction on heating. A coatingof insulation 64 is applied over the section 60 only and the heatingelement layer 66 is coextensive with the insulating layer 64. Thus allbending takes place in the section 60 in response to thermal heating bythe resistive element 66. The bimetal section 62 acts to compensate forchanges in ambient temperature which acts on both the sections 60 and62. By this arrangement, the response time of the relay to theapplication of a current to the heating element 66 is not affected bychanges in ambient temperature conditions.

In the embodiment shown in FIG. 5, a modification of thethermally-actuated relay is shown in which the case 10 is provided witha cover 70 in which is mounted an adjustably positioned heat sink 72.The heat sink is in the form of a mass of metal secured to the end of ascrew 74 which threadedly passes through the top 70 andis rotated by aknurled head 76. By screwing the heat sink in and out to vary thedistance between the heat sink and the heating element on the surface ofthe bimetallic strip 40, the rate of heating of the bimetallic strip canbe varied so as to change the time response of the relay.

An alternative relay design is shown in FIG. 6, in

which the bimetal strip is in the form of two sections 80 and 82 thatbend in opposite directions in response to a given temperature change.Opposite ends of the strip formed by the two sections 80 and 82 engageslots 84 in opposite ends 86 and 88 of an enclosing housing or case 90.A resistive heating layer 94 is bonded to the surface of the section 80in the same manner as described above. A moving contact 96 is providedinter mediate the sections 80 and 82 which moves between two fixedcontacts 98 and 100 positioned inside the case 90. The bimetallic stripis formed into an S-shape,

as shown, by compressing the ends together, with the moving contact 96pressing against the fixed contact 98. The natural spring effect of thecompressed S- shaped strip holds the strip in position. It should benoted that, although the ends of the strip are shown as pivoting innotches 84, either or both ends may be anchored to the housing incantilever fashion, as indicated at 28 in FIG. 1.

The S-shaped spring formed by the bimetallic strip is unstable and,in'the absence of the restraint of fixed contacts 98 and, 100, tends tosnap outwardly into a U- shape. Assuming equal but opposite bending ofeach half the strip, the 'center of the S-shaped strip is at a point ofunstable equilibrium when located on centerline 101. On either side ofthis position the straightening action of the S-shaped spring urges thecontact 96 either upwardly or downwardly. If the fixed contacts 98 and100 are spaced on either side of the equilibrium point, the device hastwo stable positions. However, if as shown in FIG. 6, contact 100 ispositioned at or slightly above the centerline 101, the only stableposition is against the contact 98. If the strip section is then heatedby the heater 94, causing the left half of the strip to bend more, theequilibrium point of the center of the strip is shifted upwardlysufficiently that the contact 96 snaps downwardly against the contact100. When the heater cools, the strip returns to its initial conditionbecause the equilibrium point moves down to the centerline 101, and thecontact 96 snaps back to the contact 98.

To provide a bistable device having the logic characteristics of alatch, the arrangement of FIG. 6 is provided with a heater on bothsections 80 and 82 of the bimetal strip; The contacts 98 and are equallyspaced on either side of the centerline 101, Le, the equilibrium pointunder ambient conditions is half way between the fixed contacts.Applying heat to one section of the strip will shift the contact 96 inone direction to its other stable position. Applying heat to the othersection of the strip will restore the contact to its initial stablecondition.

While the heater has been specifically described as formed by etching alayer of nichrome material, the resistive layer can be formed by otherwell known means, such as by vapor depositing a metallic film on theinsulating layer or by using a resistive paint, such as a carbon dagcoating. Either of these alternative arrangements provides a heatingelement which is highly flexible so as not to interfere with the normalbending of the bimetallic material with change in temperature.

While the thermo-electrical actuator has been specifically described asproviding an actuation of a switch mechanism to provide a relay, theactuator itself may be useful in other applications heretofore reservedfor solenoids and other electro-megnetic devices. The actuator is thecomplete analog to a coil/magnetic movement.

The relay is particularly useful where the speed of response is not acritical factor. Response times of the order of 1 or 2 seconds for therelay are readily provided. Where such response times can be tolerated,the device provides an extremely low cost non-inductive, non-magneticrelay mechanism.

What is claimed is:

1. An actuator mechanism comprising a bimetal strip, the bimetal stripincluding two contiguous sections having opposite bending action withchange in temperature, a thin coating of electrical insulation bonded toa portion of the surface of the strip, a thin electrically resistiveheater layer bonded to the surface of the coating and being positionedon only one of said sections of the bimetal strip, the insulationcoating being sufficiently thin to provide good thermal conductivitybetween the resistive layer and the bimetal, means for connecting asource of electrical current across the resistive layer, and meanssupporting the strip at one end in cantilever fashion, the other end ofthe strip being movable with bending of said one section by theresistive layer.

2. Apparatus of claim 1 including a heat sink adjustably positionedadjacent to but spaced from the resistive layer for absorbing heatgenerated by the resisitive layer.

3. Apparatus of claim 1 further comprising a pair of electricalcontacts, and means actuated by movement of the bimetal strip foropening and closing said contacts to make and break an electricalcurrent path.

4. An actuator mechanism comprising a bimetal strip, a thin coating ofelectrical insulation bonded to a portion of the surface of the strip, athin electrically resistive heater layer bonded to the surface of thecoating, the insulation coating being sufficiently thin to provide goodthermal conductivity between the resistive layer and the bimetal, meansfor connecting a source of electrical current across the resistivelayer, means supporting the strip at one end in cantilever fashion, theother end of the strip being movable with bending of the bimetal strip,a flat spring anchored on one end, a pivot engaging the spring at anintermediate position, the pivot being positioned to cause the flatspring to bow, means engaging the moving end of the bimetal with theflat spring at a point between the pivot and anchored end, the bimetalstrip urging the flat spring past the center line between the anchor endand the pivot, a pair of electrical contacts, and means opening andclosing the electrical contacts with rotation of the spring about saidpivot.

5. The apparatus of claim 4 wherein the flat spring has a sectionadjacent the anchored end that has a higher spring constant than theremaining section adjacent the pivoted end.

6. An actuator mechanism comprising a bimetal strip, a thin coating ofelectrical insulation bonded to a portion of the surface of the strip, athin electrically resistive heater layer bonded to the surface of thecoating, the insulation coating being sufficiently thin to provide goodthermal conductivity between the resistive layer and the bimetal, meansfor connecting a source of electrical current across the resistivelayer, means constraining the ends of the bimetal strip to compress thestrip into an S-shape in the center portion of the strip, the center ofthe compressed strip having an unstable equilibrium point correspondingto the maximum energy state of the compressed strip, the equilibriumpoint being shifted laterally by heating and cooling of the bimetal, andspaced limit means on either side of the center of the strip forrestricting lateral movement of the center of the strip in eitherdirection, the spacing being such that the equilibrium point shiftsbeyond the limits of the limit means with heating and cooling of thebimetal.

7. Apparatus of claim 6 wherein the limit means include a pair of fixedelectrical contacts positioned on opposite sides of the strip, and acontact carried by the strip and movable with displacement of the centerof the strip into contact with either of the fixed contacts.

8. Apparatus of claim 7 wherein both fixed contacts are positioned tolimit movement of the center of the strip entirely to one side of theequilibrium point of the compressed strip at the ambient temperature.

9. Apparatus of claim 7 wherein the fixed contacts are positioned oneither side of the equilibrium point of the compressed strip at theambient temperature.

10. Apparatus of claim 9 including a second resistive heater layer, andmeans connecting a source of an electrical current, the first mentionedand second resistive layers being positioned respectively on the twoloops of the S-shaped strip.

11. A switching device comprising a bimetallic strip, means constrainingthe ends of the strip to compress the center portion of the strip intoan S-shape, the center of the compressed strip having an unstableequilibrium point corresponding to the maximum energy state of thecompressed strip, the equilibrium point being shifted by heating andcooling of the bimetal strip, spaced limit means positioned on eitherside of the center of the strip for restricting lateral movement of thecenter of the strip, and means for heating the strip sufficiently toshift the equilibrium point outside the space between the spaced limitmeans.

12. Apparatus of claim 11 wherein the limit means include a pair offixed electrical contacts positioned on opposite sides of the strip, anda contact carried by the strip and movable with displacement of thecenter of the strip into contact with either of the fixed contacts.

13. Apparatus of claim 12 wherein both fixed contacts are positioned tolimit movement of the center of the strip entirely to one side of theequilibrium point of the compressed strip at the ambient temperature.

14. Apparatus of claim 12 wherein the fixed contacts are positioned oneither side of the equilibrium point of the compressed strip at theambient temperature.

15. Apparatus of claim 11 wherein the bimetal strip has two sections oneither side of the center which bend in opposite directions with changein temperature.

16. Apparatus of claim 15 wherein the heater means heats only one ofsaid sections of the strip.

17. Apparatus of claim 15 wherein the heater means includes means forseparately heating each section of the bimetal strip.

18. Apparatus of claim 11 wherein the constraining means includes meansfor supporting at least one end of the strip in cantilever fashion.

1. An actuator mechanism comprising a bimetal strip, the bimetal stripincluding two contiguous sections having opposite bending action withchange in temperature, a thin coating of electrical insulation bonded toa portion of the surface of the strip, a thin electrically resistiveheater layer bonded to the surface of the coating and being positionedon only one of said sections of the bimetal strip, the insulationcoating being sufficiently thin to provide good thermal conductivitybetween the resistive layer and the bimetal, means for connecting asource of electrical current across the resistive layer, and meanssupporting the strip at one end in cantilever fashion, the other end ofthe strip being movable with bending of said one section by theresistive layer.
 2. Apparatus of claim 1 including a heat sinkadjustably positioned adjacent to but spaced from the resistive layerfor absorbing heat generated by the resisitive layer.
 3. Apparatus ofclaim 1 further comprising a pair of electrical contacts, and meansactuated by movement of the bimetal strip for opening and closing saidcontacts to make and break an electrical current path.
 4. An actuatormechanism comprising a bimetal strip, a thin coating of electricalinsulation bonded to a portion of the surface of the strip, a thinelectrically resistive heater layer bonded to the surface of thecoating, the insulation coatinG being sufficiently thin to provide goodthermal conductivity between the resistive layer and the bimetal, meansfor connecting a source of electrical current across the resistivelayer, means supporting the strip at one end in cantilever fashion, theother end of the strip being movable with bending of the bimetal strip,a flat spring anchored on one end, a pivot engaging the spring at anintermediate position, the pivot being positioned to cause the flatspring to bow, means engaging the moving end of the bimetal with theflat spring at a point between the pivot and anchored end, the bimetalstrip urging the flat spring past the center line between the anchor endand the pivot, a pair of electrical contacts, and means opening andclosing the electrical contacts with rotation of the spring about saidpivot.
 5. The apparatus of claim 4 wherein the flat spring has a sectionadjacent the anchored end that has a higher spring constant than theremaining section adjacent the pivoted end.
 6. An actuator mechanismcomprising a bimetal strip, a thin coating of electrical insulationbonded to a portion of the surface of the strip, a thin electricallyresistive heater layer bonded to the surface of the coating, theinsulation coating being sufficiently thin to provide good thermalconductivity between the resistive layer and the bimetal, means forconnecting a source of electrical current across the resistive layer,means constraining the ends of the bimetal strip to compress the stripinto an S-shape in the center portion of the strip, the center of thecompressed strip having an unstable equilibrium point corresponding tothe maximum energy state of the compressed strip, the equilibrium pointbeing shifted laterally by heating and cooling of the bimetal, andspaced limit means on either side of the center of the strip forrestricting lateral movement of the center of the strip in eitherdirection, the spacing being such that the equilibrium point shiftsbeyond the limits of the limit means with heating and cooling of thebimetal.
 7. Apparatus of claim 6 wherein the limit means include a pairof fixed electrical contacts positioned on opposite sides of the strip,and a contact carried by the strip and movable with displacement of thecenter of the strip into contact with either of the fixed contacts. 8.Apparatus of claim 7 wherein both fixed contacts are positioned to limitmovement of the center of the strip entirely to one side of theequilibrium point of the compressed strip at the ambient temperature. 9.Apparatus of claim 7 wherein the fixed contacts are positioned on eitherside of the equilibrium point of the compressed strip at the ambienttemperature.
 10. Apparatus of claim 9 including a second resistiveheater layer, and means connecting a source of an electrical current,the first mentioned and second resistive layers being positionedrespectively on the two loops of the S-shaped strip.
 11. A switchingdevice comprising a bimetallic strip, means constraining the ends of thestrip to compress the center portion of the strip into an S-shape, thecenter of the compressed strip having an unstable equilibrium pointcorresponding to the maximum energy state of the compressed strip, theequilibrium point being shifted by heating and cooling of the bimetalstrip, spaced limit means positioned on either side of the center of thestrip for restricting lateral movement of the center of the strip, andmeans for heating the strip sufficiently to shift the equilibrium pointoutside the space between the spaced limit means.
 12. Apparatus of claim11 wherein the limit means include a pair of fixed electrical contactspositioned on opposite sides of the strip, and a contact carried by thestrip and movable with displacement of the center of the strip intocontact with either of the fixed contacts.
 13. Apparatus of claim 12wherein both fixed contacts are positioned to limit movement of thecenter of the strip entirely to one side of the equilibriUm point of thecompressed strip at the ambient temperature.
 14. Apparatus of claim 12wherein the fixed contacts are positioned on either side of theequilibrium point of the compressed strip at the ambient temperature.15. Apparatus of claim 11 wherein the bimetal strip has two sections oneither side of the center which bend in opposite directions with changein temperature.
 16. Apparatus of claim 15 wherein the heater means heatsonly one of said sections of the strip.
 17. Apparatus of claim 15wherein the heater means includes means for separately heating eachsection of the bimetal strip.
 18. Apparatus of claim 11 wherein theconstraining means includes means for supporting at least one end of thestrip in cantilever fashion.